Coal tar enamels, asphalt, and epoxy coal tar paints have previously been used as anti-corrosive coating compositions. These coating compositions have a number of drawbacks, inter alia, they are poor in low temperature characterics such as curability, brittleness, impact resistance and flexibility. The example, epoxy coal tar paints, while having good adhesion properties, have poor coating efficiency and abrasion resistance because cure time is extensive thereby hindering the application of relatively thick coatings.
Melt coating of polyethylene has also been used to provide ferrous substrates with corrosion inhibiting coatings, however, these require large-scale equipment for application, are not practical for the application of small amounts, or onto difficultly accessible areas such as in weld zones or irregularly shaped portions of steel pipes.
Polyurethane coatings for metallic substrates are known, particularly in the automotive industry field. See, for example, U.K. Patent Application No. 2,147,910 or U.S. Pat. Nos. 4,554,188, 4,400,497 or 4,525,570. Two-component mixes for the preparation of polyurethane resins are well known. The first polyurethane resins were made by reacting together a polyol and a diisocyanate. As a result, various two-component mixes are described, for example, in U.S. Pat. Nos. 4,410,597 and 4,101,473.
The prior art is documented by patented literature describing two-component polyurethane systems comprised of a single polyol compound component and of a polyisocyanate compound component.
The prior art is also documented by references characterized by teachings of the use of a combination of polyol compounds of differing functionality, i.e. of hereinafter referred to compounded polyols, in conjunction with a given polyisocyanate compound.
U.S. Pat. No. 5,290,632 discloses a two-component polyurethane composition comprising (A) a mixture of castor oil, a polyol compound, and an elastomeric diol compound and (B) a polyisocyanate compound.
Additional references teaching the use of compounded polyols in conjunction with polyisocyanates in the preparation of polyurethane coatings include the following:
U.S. Pat. No. 4,100,010, reissued as U.S. Re. Pat. No. 33,175 which teaches the use of a mixture of polypropylene diol and polypropylene triol in conjunction with an aliphatic diisocyanate compound.
U.S. Pat. No. 3,539,424 which describes a polyurethane resin film made from two components, where the polyol component may be a mixture of polypropylene ether triol and polypropylene ether diol and where the isocyanate component is toluene diisocyanate.
Additionally, U.S. Pat. No. 4,853,054 teaches the use of a diphenylmethane diisocyanate (MDI) prepolymer in conjunction with a mixture of polyols as part of a polyurethane forming composition and therein said mixture of polyols is a trifunctional ethylene oxide capped polypropylene oxide with a difunctional ethylene oxide-capped polypropylene oxide.
Additional references teaching polyurethane systems comprising MDI/polyol combination components are found in U.S. Pat. Nos. 5,021,535, 4,855,185, 4,710,560, 4,365,025.
None of the above cited references disclose formulations which provide polyurethane products which exhibit superior corrosion properties fast cure ratio, good adhesion, adequate flexibility, low moisture transmission and no delamination when applied to a substrate.
The need has arisen therefore, for the provision of polyurethane compositions which will be characterized by the absence of the commonly found disadvantages of the prior art conventional anti-corrosion coating compositions.